How do I make a luminescent gas cloud?
Simple Sphere with Emission as a Volume would be sufficient. We will use for both methods mostly the same setup. Gradient Texture > Color Ramp > Emission shader.

Color Ramp will control color and smoothness.
Emission > Strength... Strength.

Without Emission.



They will look like gas spheres and emission will not affect other meshes.



With Emission.



This ones will affect meshes with light.